1. Technical Field
The present invention relates to a transistor, and more particularly, to a metal oxide semiconductor field effect transistor (MOSFET) and a method of fabricating the same.
2. Description of the Related Art
To achieve a higher integration of semiconductor devices, the size of the semiconductor devices needs to be reduced. Particularly, a critical dimension (CD), which generally equals a length of a gate electrode of a metal oxide semiconductor field effect transistor (MOSFET), needs to be reduced. However, a reduced CD results in a reduced channel length of a MOSFET, which causes various problems often referred to as short-channel effects.
Short channel effects deteriorate the characteristics of the MOSFET. For instance, as the channel length decreases, a threshold voltage decreases, a drain induced barrier lowering (DIBL) phenomenon occurs, and a leakage current between a drain and a source increases.
In order to prevent or alleviate the short channel effects, there is proposed a method for implanting impurities for threshold voltage adjustment over an entire surface of a channel. However, this method does not allow high integration of semiconductor devices.
Alternative methods for alleviating the short channel effects have also been proposed. For example, in order to adjust a vertical doping profile, a super steep retrograde (SSR) well or a pocket implantation may be formed. However, these methods do not prevent the decrease in the threshold voltage due to the short channel effects. For another example, halo ion implantation regions formed in lower portions of the source/drain regions may also be formed to reduce the short channel effects.
FIG. 1 is a cross-sectional view of a semiconductor device illustrating a related art method for forming halo ion implantation regions.
Referring to FIG. 1, a gate oxide 7 and a gate electrode 9 are formed on a semiconductor substrate 1. Semiconductor substrate 1 may comprise silicon. Semiconductor substrate 1 includes a low concentration impurity region 3 and a high concentration impurity region 5 acting as a source or drain region on each side of gate oxide 7 and gate electrode 9. Oxide spacers 11 are formed on sidewalls of gate electrode 9 and gate oxide 7.
To form halo ion implantation regions 13, impurities are implanted into lower portions of the source/drain regions using gate electrode 9 and oxide spacers 11 as an ion implantation mask. Halo ion implantation regions 13 prevent the reduction in the threshold voltage and preserve the channel mobility.
However, a problem with the related art halo ion implantation method is that a junction capacitance may be increased and a junction depth is affected because the halo ion implantation regions are formed in source/drain regions as well as the semiconductor substrate region. Accordingly, when the integration density is high, the threshold voltage of devices with halo ion implantation regions is difficult to control.